Optical glass fiber must be coated to protect its surface against abrasion. Since heat-cured coatings are slow curing, it has been desired to employ ultraviolet-curing coating compositions. This proved to be difficult in practice because the optical fiber must be expected to encounter a wide range of service temperatures, including very low service temperatures. The usual ultraviolet-cured coatings are either too hard initially, or become too hard at the lower service temperatures. This excessive hardness causes the difference between the thermal coefficient of expansion of the coating and the thermal coefficient of expansion of the glass to produce microbends in the fiber when low service temperatures are encountered. These microbends interfere with the capacity of the fiber to convey optical messages.
Industry experienced great difficulty in providing ultraviolet curing coatings which would have enough strength at room or expected elevated service temperature to protect the glass surface against mechanical stress without inducing microbending difficulties at low service temperature until R. E. Ansel, in Ser. No. 170,148 filed July 18, 1980, now U.S. Pat. No. 4,624,994, found that certain urethane oligomer diacrylates could be combined with appropriate mixtures of monoethylenically unsaturated monomers including a large proportion of a monomer of low glass transition temperature to provide a primer or buffer coating which could be overcoated with a stronger and harder topcoat to provide the combination of properties which was needed.
Unfortunately, the coatings disclosed in the aforesaid Ansel application are only able to resist temperatures down to around -40.degree. C., and they require overcoating. While other ultraviolet-cured coatings having better low temperature properties have been found, these are softer at room temperature, and thus more in need of overcoating.
Accordingly, one objective of this invention is to provide ultraviolet-curable coatings which combine reasonably good low temperature microbending resistance with sufficient room temperature strength to be useful in the absence of topcoating.
Optical fibers not only encounter low service temperatures, but they also encounter elevated service temperatures. Those coatings which provide good low temperature characteristics are frequently much too soft at room or elevated service temperature, and thus must be topcoated. It has therefore been found desirable to topcoat a buffer coated optical glass fiber with a tough and flexible overcoat possessing superior resistance to moisture and abrasion. To obtain the desired properties in optical glass fibers which have been buffer coated, resort has been had to the use of extruded Nylon "jacket" coatings, but these are more expensive and difficult to apply than ultraviolet-cured coatings.
It is also known to apply both the buffer coating and the topcoating at high speed using an ultraviolet-curable topcoat on top of a buffer coating which has been ultraviolet cured, but the ultraviolet-cured topcoats have not possessed the desired strength and resistance to rupture without being inadequately flexible.
Another objective of this invention is to provide ultraviolet-curable topcoatings which will substantially duplicate the properties now obtained using the extruded "jacket" coatings noted above so that high speed coating procedures can be used to economically produce buffer coated and topcoated optical glass fiber which will satisfy the demanding commercial requirements which are insisted upon.
Still another objective of this invention is to provide radiation-curable oligomers which can be used for coating optical glass fibers, as discussed above, or which can be cured with electron beam radiation to form coatings having a favorable combination of high strength and good elongation.